If I was buying a 2.5 then i would pick it up and drop it straight off for a bottom end rebuild (rods, pistons, head studs and associated gaskets etc, heads shouldn't need dooing), but i can tell you three things,
1. its pot luck if yours goes or not
2. its cheaper to sort the engine before it goes than after its gone, when they go they can in the worst case write off the whole engine
3. a properly built forged 2.5 will run 450bhp reliably for many years
then if you rally wanna go the whole hog have it closed deck convert and go for 550, like i have haha